Polymer

A second method by which monomers bond together to form polymers is called condensation. The formation of condensation polymers is more complex that the formation of addition polymers. Unlike addition polymers, in which all the atoms of the monomers are present in the polymer, two products result from the formation of condensation polymers, the polymer itself and another small molecule which is often, but not always, water. These polymers can form from a single kind of monomer, or, copolymers can form if two or more different monomers are involved. Most of the natural polymers are formed by condensation.

One of the simplest of the condensation polymers is a type of nylon called nylon 6. It is formed from an amino acid, 6-aminohexanoic acid that has six carbon atoms in it, hence the name nylon 6. All amino acids molecules have an amine group (NH₂) at one end and a carboxylic acid (COOH) group at the other end. A polymer forms when a hydrogen atom from the amine end of one molecule and an oxygen-hydrogen group (OH) from the carboxylic acid end of a second molecule split off and form a water molecule. The monomers join together as a new chemical bond forms between the nitrogen and carbon atoms. This new bond is called an amide linkage. Polymers formed by this kind of condensation reaction are referred to as polyamides. The new molecule, just like each of the monomers from which it formed, also has an amine group at one end (that can add to the carboxylic acid group of another monomer) and it has a carboxylic acid group at the other end (that can add to the amine end of another monomer). The chain can continue to grow and form very large polymers. Each time a monomer is added to the chain, a small molecule byproduct of water is also formed.

All of the various types of nylons are polyamides because the condensation reaction occurs between an amine group and an acid group. The most important type of nylon is a copolymer called nylon 66, so-named because each of the monomers from which it forms has six carbon atoms. Nylon 66 is formed from adipic acid and hexamethylenediamine. Adipic acid has a carboxylic acid group at both ends of the molecule and the hexamethylenediamine molecule has an amine group at both ends of the molecule. The polymer is formed as alternating monomers of adipic acid and hexamethylenediamine bond together in a condensation reaction and a water molecule splits away.

Nylon became a commercial product for Du Pont when their research scientists were able to draw it into long, thin, symmetrical filaments. As these polymer chains line up side-by-side, weak chemical bonds called hydrogen bonds form between adjacent chains. This makes the filaments very strong. Nylon was first introduced to the public as nylon stockings (replacing the weaker natural fiber, silk) in October, 1939 in Delaware. Four thousand pairs sold in no time. A few months later, four million pairs sold in New York City in just one day. But the new found treasure was short-lived since, when the United States entered World War II in December, 1941, all the nylon went into making war materials. Women again had to rely on silk, rayon, cotton, and some even went to painting their legs. Nylon hosiery did not become available again until 1946.

Another similar polymer of the polyamide type is the extremely light-weight but strong material known as Kevlar. It is used in bullet-proof vests, aircraft, and in recreational uses such as canoes. Like nylon, one of the monomers from which it is made is terephthalic acid. The other one is phenylenediamine.

Polyesters are another type of condensation polymer, so-called because the linkages formed when the monomers join together are called esters. Probably the best known polyester is known by its trade name, Dacron. It is a copolymer of terephthalic acid (which has a carboxylic acid at both ends) and ethylene glycol (which has an alcohol, OH group), at both ends. A molecule of water forms when the OH group from the acid molecule splits away and bonds with a hydrogen atom from the alcohol group. The new polymer is called polyethylene terephthalate or PET and can be recognized by its recycling code number 1.

Dacron is used primarily in fabrics and clear beverage bottles. Films of Dacron can be coated with metallic oxides, rolled into very thin sheets (only about one-thirtieth the thickness of a human hair), magnetized, and used to make audio and video tapes. When used in this way, it is extremely strong and goes by the trade name Mylar. Because it is not chemically reactive, and is not toxic, allergenic, or flammable, and because it does not promote blood-clotting, it can be used to replace human blood vessels when they are severely blocked and damaged or to replace the skin of burn victims.

There are other important condensation polymers that are formed by more complex reactions. These include the formaldehyde resins the first of which was Bakelite. These plastics are thermosetting plastics; that is, once they are molded and formed, they become permanently hard and they cannot be softened and remolded. Today their major use is in plywood adhesives, Melmac for dinnerware, Formica for table and counter tops, and other molding compounds.

Polycarbonate polymers are known for their unusual toughness, yet they are so clear that they are used for "bullet-proof" windows and in visors for space helmets. The tough, baked-on finishes of automobiles and major appliances are cross-linked polymers formed from an alcohol, such as glycerol, and an acid, such as phthalic acid, and are called alkyds. Silicone oils and rubbers are condensation polymers that have silicon rather than carbon as part of their structural form. These compounds are generally more stable at high temperatures and more fluid at low temperatures than the carbon compounds. They are often used for parts in space ships and jet planes.